This invention relates to optical waveguide systems and devices, and more particularly to systems and devices which are intended to switch signals of selected optical wavelengths from one waveguide to another.
Systems for communicating or processing data at optical wavelengths are in wide use, and the number of applications for such systems are increasing rapidly. A basic component for such systems is the optical switch, for transferring an input signal to either of two lines (such as optical waveguide fibers). There are a number of corollaries of electromechanical switches that are known, operating on the principle of displacing a conductive element so that it completes an optical circuit with one line or another. However, even though much work has been done on improving alignment and matching techniques, the transition of a signal from one element to another inevitably introduces losses. The techniques for reducing those losses introduce undesirable and sometimes unacceptable increments of cost.
Furthermore, modern communication and other systems take advantage wherever economically feasible of the broadband capabilities of optical waveguides, as by the use of wavelength division multiplexing to propagate a number of wavelength separated signals on one optical fiber. In the standards that have evolved for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM), there are specific, closely spaced channels at designated wavelengths. Wavelength sensitive devices, such as add/drop filters and couplers, are utilized to extract signals in specific channels from, or add signals in specific channels to, the multiplexed signal. In some modern systems, therefore, switching a specific wavelength out of a WDM line requires both a wavelength selective coupler and a conventional switch. The added elements are not only more costly, but the losses introduced where matching or transitions are needed become unacceptable, particularly if a number of switches are to be used in the system. There is therefore a need for a wavelength selective optical switching system that has negligible loss and can provide a high degree of selectivity for a specific channel.
Devices and systems in accordance with the invention employ a grating assisted optical coupler having a length of small diameter (e.g. less than 15 microns) fiber that includes a Bragg grating that is reflective at wavelengths of a given periodicity. This length of fiber is held under precise tension, to tune the. periodicity of the refractive index variations to the specific wavelength desired. Under these circumstances, input signals at the selected wavelength are reflected back to a drop port, until the periodicity of the grating is varied. In the present example this is done on demand by stretching the small diameter section a small amount, shifting the effective wavelength to outside the band of the target channel. Thus the signal passes through the grating region to a different output fiber instead of being reflected. In either event, fiber continuity is not affected and losses are very low. By this arrangement, the small diameter section containing the grating is stretched only within its elasticity limits, and only by a small movement, which is imparted very rapidly, achieving desired switch response times.
In a more specific example of a wavelength selective optical switch, the unit comprises an add/drop coupler having an input port and drop port on one side of a waist region of small diameter containing the Bragg grating, and also having throughput port and add port terminals on the opposite side of the Bragg grating region. The waist region is held under controlled tension in a temperature compensated structure, and also tuned by tension to selected grating periodicity. This grating assisted coupler includes biconical tapered sections leading from each of the ports to the waist region, at which two greatly reduced diameter fiber elements are fused together. To stretch this reduced diameter waist region rapidly in controlled fashion, a small ferromagnetic element is attached to a tapered section at a distance from the Bragg grating. The ferromagnetic metal element and the fiber are deflected toward an associated electromagnet having a pole tip of the order of 1 mm away, stretching the Bragg region. This is realized within a switching time of less than 10 milliseconds. Thus selected wavelength signals that normally would be reflected back from the Bragg grating region to the drop port are transferred through the waist region to the output port. This coupler further offers the advantage of enabling addition of a signal of the selected wavelength at the add port, even though a multiplexed signal at the same wavelength is concurrently being dropped. Further mechanical stability and switch life are enhanced by incorporating a buffer element between the electromagnetic pole tip and the metal on the tapered fiber section which is deflected during switching.
Other aspects of this combination reside in the fact that the mounting structure which holds the ends of the span containing the Bragg grating compensates for temperature variations that would otherwise affect wavelength sensitivity. The change in periodicity, when the switch is activated, does not affect passband characteristics, so that there is no waste of spectrum or interference.
Only a small deviation in length is needed to change the reflection characteristic of the Bragg grating, and so a number of other expedients can be employed to effect switching in accordance with the invention. For example, a small deflection movement can be introduced even more rapidly by employing a leverage arm whose free end shifts the waist region laterally with a small angle movement of a rotary actuator at the opposite end. Other actuating forces may alternatively be generated using electronic, electromagnetic or electrostatic effects. Extremely high rate movements through limited distances are possible with capacitive devices, cantilevered elements, Hall effect devices and piezoelectric devices, for example. Furthermore, a small voice coil mechanism might alternatively be used to provide higher speed movement and faster switching.
Methods in accordance with the invention include the steps of establishing a narrow band of wavelength selectivity, using tension to maintain the selectivity and then varying the responsiveness by mechanically stretching until the periodicity has been shifted outside the selected wavelength band. Further, the methods employ tuning and temperature stabilization to enhance the precision and long life properties of the device.